A “cooperative communication” technology in a plurality of cells is considered as a method for improving the reception quality of a mobile station at the edge of a cell in Long Term Evolution-Advanced (LTE-A) by the 3rd Generation Partnership Project (3GPP). The cooperative communication is a technique in which a plurality of points (base stations or antennas) cooperates with each other to communicate with one or more mobile stations. For example, the same data is transmitted from the points to a terminal in the cooperative communication in the downlink. The cooperative communication in the downlink increases the gain by the effect of spatial diversity. The increase improves the reception quality of a mobile station at the edge of a cell. The cooperative communication in LTE-A is referred to as Coordinated Multi-Point (CoMP) communication.
Herein, all of the cells around a mobile station are candidates for the cell of the cooperative communication (hereinafter, sometimes referred to merely as a “candidate cell”). Selecting a cell to be related to the cooperative communication (namely, a cooperative cell) from the candidate cells increases the number of processes for determination in the mobile station and the amount of information about the determination result from the mobile station to the base station. In light of the foregoing, a small number of cells are previously selected as the candidate cells. A method for selecting the candidate cells is that Reference Signal Received Power (RSRP) of the reference signal to be transmitted to the mobile station in each cell is used. In LTE-A Rel. 11, the candidate cells are selected with the RSRP determined with a Cell-specific Reference Signal (CRS) and sets of the selected candidate cells, namely, CoMP Resource Management (CRM) sets are formed.
Note that the “cell” is defined based on the “communication area” and “channel frequency” of a base station. The “communication area” can be the whole of the area that the radio waves transmitted from the base station reach (hereinafter, sometimes referred to as a “range area”) or can be divided areas obtained by dividing the range area (so-called, sectors). The “channel frequency” is a unit of frequencies that the base station uses for the communication, and is defined based on the center frequency and the bandwidth.
Alternatively, it is considered for LTE-A Rel. 12 that three-dimensional beam-forming in which vertical beam-forming is added to horizontal beam-forming is used in transmission from a base station.
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2012-135051, in Chao Shen, Tsung-Hui Chang, Member, IEEE, Kun-Yu Wang, Zhengding Qiu, and Chong-Yung Chi, Senior Member, IEEE, “Distributed Robust Multicell Coordinated Beamforming With Imperfect CSI: An ADMM Approach”, IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. 60, NO. 6, June, 2012, and in Pengfei Xia, Shengli Zhou, and Georgios B. Giannakisl, Dept. of Electrical and Computer Engr., University of Minnesota 200 Union St. SE, Minneapolis, Minn. 55455, “MIMO OFDM with ST Coding and Beamforming Adapted to Partial CSI”, 2003 Conference on Information Sciences and Systems, The Johns Hopkins University, Mar. 12-14, 2003.
When performing beam-forming transmission, the base station uses a plurality of antennas to form transmission beams to a mobile station. When the mobile station determines RSRP using a reference signal transmitted from one of the antennas, the RSRP is lower than the received power of the data signal transmitted by the beam-forming transmission. It is assumed that the beam-forming is used for the cooperative communication. In such a case, when the RSRP of a reference signal transmitted from one of the antennas is used to select a candidate cell, the candidate cell is selected based on the received power different from the received power when the data signal is actually transmitted. This adversely reduces the gain in the cooperative communication. The adverse reduction becomes further pronounced in three-dimensional beam-forming in comparison with in beam-forming in a horizontal direction or a vertical direction (namely, in two-dimensional beam-forming) because the gain is further increased in three-dimensional beam-forming.